Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft drives the generator rotor either directly (“directly driven”) or through the use of a gearbox.
Pitch systems may be employed for adapting the position of a wind turbine blade to varying wind conditions. In this respect, it is known to rotate the position of a wind turbine blade along its longitudinal axis in such a way that it generates less lift (and drag) when the wind speed increases. In this way, even though the wind speed increases, the torque transmitted by the rotor to the generator remains substantially the same. It is furthermore also known to rotate wind turbine blades towards their stall position (so as to reduce the lift on the blades) when the wind speed increases. These wind turbines are sometimes referred to as “active-stall” wind turbines. Pitching may furthermore also be used for rotation of the blades towards their vane position, when a turbine is temporarily stopped or taken out of operation for e.g. maintenance.
Pitch systems generally comprise an electric or hydraulic motor which, through the use of reduction gearing (sometimes referred to as a “reductor”, or “reduction gear”), drives an actuating gear. Said actuating gear (pinion) is generally arranged to mesh with an annular gear provided on the wind turbine blade to set the wind turbine blade into rotation. Other actuating mechanisms operated by a pitch motor are also known.
It is further known to provide an individual pitch system (comprising a separate motor and separate control) for each individual wind turbine blade of a rotor. It is also known to provide a common pitch system wherein the pitch angle of the blades is the same for all blades on a rotor. Such a common pitch system may comprise a single motor or may comprise a plurality of motors, one for each blade.
A control strategy of a pitch system that is often employed in variable speed wind turbines is to maintain the blade in a default pitch position at wind speeds equal to or below nominal wind speed (for example, approximately 4 m/s-15 m/s). Said default pitch position may generally be close to a 0° pitch angle. The exact pitch angle in or below nominal wind speed conditions depends however on the complete design of the wind turbine. Above the nominal wind speed (for example from approximately 15 m/s-25 m/s), the blades are rotated to maintain the aerodynamic torque delivered by the rotor substantially constant. When the wind turbine is not operating, the blades may assume a vane position (e.g. at or around 90° pitch angle) to minimize the loads on the blades. During most of the wind turbine's life, a blade may however be in the same pitch position which is that at or below nominal wind speed. The nominal wind speed, cut-in wind speed and cut-out wind speed may of course vary depending on the wind turbine design.
During operation of the wind turbine, forces may be acting on the blades that result in a constantly varying torque around the blade's longitudinal axis. These forces may include the aerodynamic torque around the longitudinal axis of the blade. Furthermore, since the blade's centre of mass is usually not located exactly on its rotating axis, the weight of the blade may exercise an additional torque around the blade's longitudinal axis. Both these forces are non-constant, largely cyclical and tend to rotate the blade out of the position determined by the pitch control system.
When a pitch system involving gearing is used, the varying torque may result in flanks of the teeth of the actuating gear (pinion) and annular gear repeatedly touching each other. Such repetitive contact between teeth may remove thin metallic particles, and may create a tooth print in the contacting flanks of the gear and the pinion. This repetitive contact may thus lead to fretting corrosion and premature wear. Since the pitch position at or below nominal wind speed is the prevailing position for most of the life of most wind turbines, the repetitive contact between the teeth and the corresponding consequences are usually concentrated on the same teeth.
Some solutions for these problems are known. It is e.g. known to provide an automatic lubrication system to try and prevent fretting corrosion. For example, DE202005014699U and EP1816346 provide such lubrication systems. These lubrication systems may help to reduce fretting corrosion to a smaller or larger extent, but do not combat or resolve the problem underlying the corrosion, namely the teeth flanks contacting each other. Further, once the teeth contacting the pinion are damaged, the whole annular gear needs to be replaced.
There still exists a need to achieve an electromechanical pitch drive system which can easily extend the life of the annular gear and the driving pinion and is also cost-effective.